1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus represented by a copying machine and a printer, and to a developing device adaptable to the image forming apparatus or a powder stirring device.
2. Description of the Related Art
In an electrophotographic image forming apparatus, such as a copying machine and a printer, a photoconductive drum is used extensively as an image carrier. A normal image forming operation with the use of the photoconductive drum is as follows. The peripheral surface of the photoconductive drum is charged uniformly to specific potential by the charging device, and an electrostatic latent image of a document image is formed on the peripheral surface of the photoconductive drum by optically attenuating the potential in part through irradiation of light to the peripheral surface from an LED or the like in the exposing device. A toner image is formed by developing the electrostatic latent image by the developing device. After the toner image is transferred onto a sheet, toner particles remaining on the peripheral surface of the photoconductive drum are cleaned by the cleaning device, and static eliminating light is irradiated to the surface from a static eliminating device for eliminating static electricity from the charged surface to prepare for the following image forming operation.
The developing device serving as a main portion of the image forming operation as described above is configured in such a manner that a specific amount of toner particles are constantly accommodated inside the developing container. When toner particles are consumed and a remaining amount becomes low, toner particles are additionally replenished. In this instance, toner particles are generally put into the developing container from a toner replenishing container disposed above the developing device by gravitational action or with the use of a transportation member.
Because the developing container accommodates the developing roller, it is normally of an elongate shape that matches the shape of the developing roller. To enable a high-quality image to be formed, it is necessary that toner particles accommodated inside the developing container are distributed homogeneously along the longitudinal direction of the developing roller inside the developing container. To this end, the developing device includes a stirring (transporting) member that rotates about the shaft axis and widely adopts a method for distributing toner particles homogeneously in the longitudinal direction inside the developing container by a rotating operation of the stirring (transporting) member.
Examples of such a developing device are described in JP-A-2001-100500 (Reference D1) and JP-A-11-311895 (Reference D2). Both the developing devices described in References D1 and D2 include a stirring (transporting) member provided with a sheet member (stirring blade) having openings therein, and transport toner particles toward the developing roller with stirring by means of the stirring (transporting) member.
The developing device described in Reference D1 is configured so as to be able to supply the developing roller with toner particles in association with rotations of the stirring and transporting member in a stable manner without experiencing large resistance by using two sheet members provided with plural openings in a staggered arrangement. With the stirring and transporting member configured in this manner, toner particles are thought to migrate in the longitudinal direction inside the developing container owing to the openings in a staggered arrangement. However, the sheet members and the openings therein are not configured in a satisfactory manner in terms of allowing the toner particles to migrate inside the developing container. It is therefore difficult to distribute the toner particles homogeneously to a satisfactory extent across the entire region in the longitudinal direction inside the developing container. Consequently, a large amount of toner particles concentrate in a region near the supplied point in the developing container, such as the toner replenishment port, and toner particles that have been left unused over a considerable time may possibly accumulate in the opposing region. Hence, an image defect is likely to occur when such accumulated toner particles in a deteriorated condition are supplied to the developing roller.
The developing device described in Reference D2 is configured in such a manner that toner particles are gathered at the center portion in the shaft axis direction of the developing roller in association with rotations of the stirring and transporting member by sheet members provided with plural parallelogram-shaped openings. With the stirring and transporting member configured in this manner, however, a large amount of toner particles are gathered at the center portion in the shaft axis direction of the developing roller, and toner particles that have been left unused over a considerable time may possibly accumulate at the both ends. Hence, an image defect is likely to occur at the both ends of the developing roller when such accumulated toner particles in a deteriorated condition are supplied to the developing roller.